This application relates to an improvement in a process for production of 1,2-dichloroethane by reaction of chlorine and ethylene in a liquid medium at temperatures between 85.degree. and about 160.degree. C. One process of this type is disclosed, for instance, in U.S. patent application No. 368,569 of Ramsey G. Campbell, filed June 11, 1973 and entitled "Process". A corresponding application has been published in Germany, Offenlegungsschrift 2,427,045, on Jan. 2, 1975. The disclosure of the said German patent application is hereby incorporated by reference into the present application.
Another process of this general type is disclosed in British Pat. No. 1,231,127 of Solvay et Cie.
In the process as described in German application No. 2,427,045 and the corresponding U.S. application, the liquid medium for the chlorination reaction comprises a liquid chlorinated hydrocarbon having two carbon atoms, or mixtures of two or more such compounds. Preferably, the liquid medium comprises 1,2-dichloroethane, 1,1,2-trichloroethane, or a mixture of these two compounds.
In one embodiment disclosed in the said German application, as well as in British Pat. No. 1,231,127, a stream of 1,2-dichloroethane recovered from the fractionation section of a process for pyrolysis of 1,2-dichloroethane to produce vinyl chloride, is introduced into the chlorination reaction system. In the process of the German application, the 1,2-dichloroethane is introduced as make-up for the circulating liquid medium. In addition, a fractionation column associated with the chlorination reactor serves to fractionate the 1,2-dichloroethane in the chlorination reactor, as well as such 1,2-dichloroethane as may be recycled from the pyrolysis process, or other sources, to remove impurities therefrom. The purified 1,2-dichloroethane from this fractionation column can be returned to the pyrolysis furnace.
The dichloroethane stream being recycled from the pyrolysis fractionation section to the chlorination reactor often contains minor but significant amounts of chloroprene and may also contain small amounts of other chlorinated C.sub.4 hydrocarbons. For example, this stream may contain from 0.01 to about 0.3 mole percent chloroprene. This chloroprene can polymerize further in the process, particularly in the overhead of fractionation columns, resulting in plugging of the columns and/or associated lines.
The aforesaid German patent application discloses that this polymerization can be prevented by subjecting the recycle dichloroethane stream to a pre-chlorination step for chlorination of the chloroprene prior to introducing this recycle stream into the main chlorination reactor system. The chloroprene is converted to heavier boiling chlorinated compounds which will not polymerize and which are said to be separated either in the fractionation column associated with the main chlorination reactor (in the German application, in line 19, from the bottom of the fractionation column) or from the bottom of the chlorination reactor itself (in line 33).
British Pat. No. 1,266,676 of Knapsack A. G. discloses a process for removing products such as chloroprene from 1,2-dichloroethane in a combination process in which ethylene is first chlorinated to produce dichloroethane, the dichloroethane is cracked to produce vinyl chloride, and cracked dichloroethane can be recycled to the chlorination reactor. The chloroprene is removed from the dichloroethane (to prevent plugging up of equipment) by chlorinating it in the presence of ferric chloride to form a high boiler with respect to dichloroethane, and which can be readily separated from it. The patent discloses three methods of chlorinating the chloroprene: (a) introducing the dichloroethane containing chloroprene into the main chlorination reactor, in which the chloroprene will be chlorinated by chlorine and ferric chloride simultaneously with the chlorination of ethylene to produce dichloroethane; (b) chlorinating at least a portion of the overhead from the light ends column to convert the chloroprene to higher boiling compounds, and recycling the chlorinated overhead to the column, the higher boiling compounds being removed at the bottom and passed to a heavy ends column; and (c) chlorinating the overhead of the light ends column and recycling the chlorinated overhead to the column, with removal of dichloroethane as a side-stream from this column. The main chlorination reactor operates a temperature of about 50.degree. C.
It has now been found that introduction of either chloroprene (as proposed in British Pat. No. 1,266,676) or chlorinated derivatives of chloroprene (as proposed in German application No. 2,427,045) into a chlorination reactor operating at a temperature of above 85.degree. C., results in a loss of yield of 1,2-dichloroethane based on chlorine at temperatures above 85.degree. C., and particularly between 85.degree. and about 160.degree. C. Surprisingly, in the main chlorination reactor, the chloroprene is not merely chlorinated to the next highest boiling derivative (mainly trichlorobutenes), but is further chlorinated to form higher boiling derivatives which may range up to fully chlorinated butane. Similarly, partially chlorinated derivatives of chloroprene are further chlorinated under these conditions to more highly chlorinated compounds. These more highly chlorinated compounds represent a yield loss of chlorine as they are removed from the chlorination reactor as heavy ends and disposed of as waste. The German patent application suggests that up to 3 moles of chlorine per mole of chloroprene may be used in the pre-chlorination step before introducing the recycled dichloroethane through the main chlorination reactor. The use of 3 moles of chlorine per mole of chloroprene represents an excessive use of chlorine in the process and, in addition, the more highly chlorinated compounds may be still further chlorinated in the main chlorination reactor, using up additional chlorine.
The introduction of the partially chlorinated recycle 1,2-dichloroethane into the fractionation column associated with the main chlorination reactor, rather than into the reactor itself, as suggested in the prior art, does not alleviate the situation. The partially chlorinated derivatives of chloroprene do not leave the column in the overhead or side streams; rather they pass downwards through the column, ultimately entering the chlorination reactor, and are further chlorinated.
It is an object of the present invention to provide an improved process for the removal of chloroprene from 1,2-dichloroethane which is being recycled to a reactor for chlorination of ethylene at a temperature of above 85.degree. C.
A further object of the present invention is to provide a process for minimizing chlorine losses in a process of chlorinating ethylene utilizing chlorine in which 1,2-dichloroethane is to be recycled to the reactor.
Further objects and advantages of the invention will become apparent from the following description of the invention.